The present invention is directed to robust planar photonic bandgap structures. In particular, the present invention is directed to robust planar photonic bandgap structures that, in connection with guiding radiation, control (for example, minimize, reduce or increase) radiation losses by employing layered dielectric structures. The present planar photonic bandgap structures can be used in connection with waveguides, waveguide bends, waveguide crossings, filters, switches and fiber-coupling structures.
The propagation of electromagnetic radiation (that is, visible, infrared, ultraviolet, TeraHertz, millimeter wave and microwave radiation) can be dramatically altered in periodically patterned devices. Such devices may comprise discrete regions of materials where each region possesses a distinct index of refraction, or regions of materials in which the indices of refraction vary continuously throughout each region. If the index contrasts (for example, the differences between the indices of refraction for the discrete regions) are sufficiently large, an optically opaque material can be formed, even though the device is composed of transparent dielectric materials. The properties of such materials have attracted great scientific interest over the last ten to fifteen years, and are considered likely candidates for applications involving telecommunications, optical signal processing and integrated optical circuits. The dielectric properties of those materials are commonly analogized to the electronic properties of crystals, which are characterized by, among other things, what is known as the forbidden energy gap. That analogy has led to the term xe2x80x9cphotonic bandgapxe2x80x9d (PBG). The PBG is a range of frequencies over which electromagnetic radiation is unable to propagate, and a PBG structure is a structure that exhibits a photonic bandgap. To achieve a photonic bandgap for electromagnetic radiation at a radial frequency xcfx89, it is necessary to achieve a structure or material with a period on the order of the wavelength xcex, where xcex=2xcfx80c/xcfx89 and c is the speed of light in a vacuum. It is generally understood that a PBG structure or material is a structure or material through which electromagnetic radiation, at any frequency in the photonic bandgap, is unable to propagate in any direction and at any polarization. A photonic bandgap that exists irrespective of the direction or polarization of the electromagnetic radiation is known as a full photonic bandgap. Of course, it may be useful in practical applications to restrict the scope of the PBG (for example, to only one polarization), assuming that the material would be used only with light at a certain polarization.
Achieving a full photonic bandgap theoretically requires that the material be patterned in all three dimensions (that is, height, length and width). Such materials have been shown to function in the microwave region, and it has been suggested that they can function at optical frequencies. However, several challenges remain to be overcome: fabrication of uniform photonic bandgap material, patterning the material and adapting the material to applications such as waveguiding. Those challenges are at least initially attributable to the difficulty encountered in attempting to pattern materials in three dimensions on the scale of optical wavelengths of interest in telecommunications, which wavelengths are on the order of 1.5 xcexcm.
One alternative to a full, three-dimensionally patterned PBG material is a patterned planar material. An example of such a patterned planar material is illustrated in FIG. 1, which shows a uniform planar PBG structure. As shown in FIG. 1, a planar PBG is patterned, for example, by chemical or other etching, with a periodic array of holes. In such a patterned device, it is possible to achieve a bandgap for light propagating at any direction in the plane and for any polarization. Such a patterned device can be characterized as a restricted version of a full, three-dimensional PBG. In a planar PBG structure, confinement of light within the plane of the layers is normally required, and is achieved by suitable choices for the indices of refraction of the constituent layers. As used herein, the phrase xe2x80x9cindex of refraction profilexe2x80x9d is a characterization of the relationship between indices of refraction and corresponding depths of the material(s) at issue. It is generally known in the field of planar photonic bandgap structures that confinement of light to patterned layers is normally achieved where the index of refraction profile within the patterned layers features indices of refraction that are all or substantially all higher than the indices of refraction in the index of refraction profiles for the substrate and superstrate. As used herein, the term xe2x80x9csubstratexe2x80x9d means an unpatterned layer or layers that are underneath the patterned region and that affect the propagation of the optical mode. Such an unpatterned layer is to be contrasted with a physical support, which provides structural stability, mounting and the like, and which plays no significant role in determining the optical propagation properties of electromagnetic radiation modes in the PBG structure. As used herein, the term xe2x80x9csuperstratexe2x80x9d means an unpatterned layer or layers that are above the patterned region and that affect the propagation of the optical mode.
For the structure of FIG. 1, for example, confinement in the vertical direction is achieved by having an index of refraction profile in the planar PBG layer that features indices of refraction that are all or substantially all higher than the indices of refraction in the index of refraction profiles for the air superstrate and the unpatterned substrate. The disadvantage of the planar PBG over the three-dimensional PBG is the potential for radiation losses in the planar configuration due to out-of-plane scattering.
In some cases, the substrate and/or superstrate comprise air. Where both the substrate and superstrate comprise air, a free-standing xe2x80x9cmembrane structurexe2x80x9d is formed, as shown in FIG. 2. As a practical matter, however, and as further shown in FIG. 2, a substrate comprising air necessarily has at least a second layer. Others have failed to recognize or appreciate the significance of that second layer(s) in controlling radiation loss. The structure of FIG. 2 also has a number of mechanical disadvantages, primarily relating to fragility and poor heat conduction away from the active PBG layer.
Importantly, PBG structures do support the propagation of electromagnetic radiation at frequencies outside the photonic bandgap. FIG. 1 illustrates light propagating in a PBG structure in such an allowed mode. Such allowed-mode propagation may be useful in applications such as beam collimation, prism-like refraction, and others. In such applications, it is important to control (for example, minimize, reduce or increase) radiation losses during propagation.
In addition to the above-mentioned applications, there are a number of other possible applications (for example, guiding light) for planar PBG structures that feature intentionally disturbed periodic structuring. FIG. 3, for example, shows such a defect waveguide based on the structure of FIG. 1, where a line of holes has been omitted during the patterning process. Analogous waveguide structures exist in the membrane geometry of FIG. 2, and are not illustrated here. In a waveguide such as that illustrated in FIG. 3, the surrounding undisturbed PBG regions are normally designed such that, at the desired frequency of operation, a photonic bandgap exists, thereby preventing light from entering the surrounding regions. In this way, light that is coupled into the waveguide (for example, from an external source) will remain in the waveguide. A key requirement for the suitability of such waveguides in practical applications is that they not suffer from large radiation losses.
The concept of guiding electromagnetic radiation (that is, light) in planar PBG structures, as described above, has led to interest in guiding light around bends of various angles. By way of example, FIG. 4 shows a 90-degree bend achieved by removing a series of holes from the periodic structure. For light transmitted around a bend to be useful, there should be a sufficiently high level of transmission of that light around the bend. Achieving a sufficiently high level of transmission requires (1) low reflection of light back along the incoming path and (2) low radiation (that is, light) losses caused by the structure.
Another planar PBG structure of interest is a waveguide crossing, which is illustrated in FIG. 5. In that device, it is desirable to achieve high transmission of light from an input waveguide to its matching output waveguide, while minimizing the unwanted transfer of energy to the second waveguide being crossed. Minimizing the unwanted coupling of energy can be achieved by suitable design of the crossing region. Suitable design of the crossing region includes controlling radiation loss as light passes through the crossing region from each input waveguide to its matching output waveguide.
Another generally known device in the field of planar photonic bandgap structures is a resonant cavity. A resonant cavity (sometimes referred to as a two-dimensional microcavity) can be used in filtering or active light-emitting devices to accomplish optical feedback. Others have attempted to control radiation losses in resonant cavities by surrounding a PBG region with air.
Referring to FIG. 6, a schematic planar PBG dispersion diagram shows the energies of electromagnetic modes as a function of the component of their wavevector in the plane of the PBG structure. FIG. 6 shows the dispersion curves for a particular polarization (for example, transverse electric (TE), wherein the electric field is polarized parallel to the plane of the PBG structure layers). The labels on the x-axis (xcex93, X, M) indicate symmetry points in different directions.
The electromagnetic modes in a planar PBG structure can be understood with reference to FIG. 6. The first type of mode shown in FIG. 6 is a bound mode, which is confined to the PBG structure. That mode is shown in FIG. 6 by the solid lines. In a perfectly uniform and infinite planar PBG structure, the bound modes suffer no or essentially no radiation losses. In the presence of unintentional defects or imperfections, such as non-uniform holes or non-uniform spacing, however, the bound modes can suffer radiation losses. Likewise, in a device in which defects or imperfections are intentionally introduced (see, for example, FIGS. 3-5), the presence of defects spoils the periodicity, thereby causing radiation losses.
The second type of mode shown in FIG. 6 is a resonant mode, which is specifically shown by the dashed lines. In contrast to a bound mode, a resonant mode can be phase matched to freely propagating electromagnetic modes in the substrate, the superstrate or both. That potential for phase-matched coupling is graphically depicted by the light line in FIG. 6 (that is, by the dot-dashed line in FIG. 6). That light line indicates the dispersion relation (that is, the relation between frequency and wavevector) for light in the substrate and superstrate. The light line is described by xcfx89=ck∥/n, where n is the index of refraction of the substrate or superstrate, as appropriate, and k∥ is the in-plane wavevector. In FIG. 6, the indices of refraction for substrate and superstrate are assumed equal, so the light lines are coincident in all directions. Where the substrate and superstrate are different materials (that is, materials with different indices of refraction), there will be two distinct light lines in any direction. The transition from bound to resonant modes can be deduced from the position of the mode with respect to the light line, and as a bound mode crosses the light line, phase-matched coupling becomes possible. Such potential for coupling changes the modes from being bound (that is, suffering no radiation losses in a perfect defect-free structure) to being resonant.
A resonant mode, such as that shown at point (a) in FIG. 6, may propagate in a manner quite similar to that of the bound mode indicated by point (b) in FIG. 6, but will suffer from radiation losses even in an ideal planar PBG. The magnitude of radiation loss for a resonant mode is characterized by the quality (Q) factor of the mode, where Q is the ratio of the total energy stored in a mode to the rate of energy loss from the mode. A low radiation loss structure will have a high Q. Conversely, a high radiation loss structure will have a low Q.
Because of their inherent radiation losses, resonant modes might appear to be less desirable than bound modes for at least some applications. Resonant modes can be used, however, in any application in which light must be coupled into or out of the planar PBG structure (for example, into photodetectors or from optical sources), or in any application that involves intentional defects such as waveguide bends, resonators, etc. For example, many telecommunication applications in which optical signals are transmitted from one location to another within an optical integrated circuit utilize waveguide bends and/or resonators. Effectively controlling (for example, sufficiently reducing or minimizing) the radiation losses in resonant modes can result in an increased operational bandwidth for that circuit.
The dispersion curves for the bound and resonant modes, shown by the solid and dashed lines in FIG. 6, respectively, define the propagation of light in the planar PBG structure. For example, FIG. 6 shows that the band of energies indicated by the gray shaded region is forbidden (that is, there are no modes at that energy for the directions shown). A photonic bandgap occurs when this forbidden region is preserved for any direction of propagation in the plane. FIG. 6 illustrates the dispersion along two specific directions, and a thorough check of the behavior along the other directions is required to ensure a bandgap does exist.
While bound and resonant modes are useful, as outlined above, a third class of modes, continuum modes, is not. Continuum modes represent electromagnetic modes that propagate essentially unaffected by the PBG structure. Those modes resemble simple plane waves propagating in the superstrate (for example, in the air above the structure) or in the substrate. Those modes can exist at any frequency above the light line, as indicated by the cross-hatched region in FIG. 6. Unlike resonant and bound modes, which occur only at frequencies determined by a particular PBG structure, continuum modes can exist throughout the entire cross-hatched region. Continuum modes are the primary cause of radiation loss in two-dimensional planar photonic crystals that contain defects or imperfections, whether intentionally introduced or unintentionally caused by fabrication errors. Those defects or imperfections disrupt the periodic symmetry of the crystal, introducing substantial in-plane momentum to pure bound or resonant modes, coupling them to the continuum states, and causing radiation losses, as illustrated schematically in FIGS. 7 and 8.
In the field of planar photonic bandgap structures, there is, generally speaking, a lack of effective methods for assessing propagation properties of structures and guiding radiation. Indeed, assuming, as an initial matter, that propagation properties are properly measured and problems, such as radiation loss, are properly recognized, approaches taken by others to guide radiation and control radiation losses have been problematical, impractical or ineffective in one way or another or for one reason or another. For example, methods for controlling radiation losses that occur in conventional refractive index planar waveguides are generally known. One such method involves surrounding an unpatterned region with regions of planar PBG material. A serious problem with that method is that its implementation is often based upon calculations for planar PBG materials that are assumed to form structures that extend infinitely in the direction normal to the plane of the layers. Such two-dimensional calculations are inherently incapable of determining the losses of finite planar PBG structures, leading some researchers to conclude, erroneously, that such structures do not suffer radiation losses.
Planar PBG structures incorporating intentional defects (for example, holes) in which the hole size is not uniform throughout the structure are generally known, as is a planar PBG waveguiding structure formed from a mesh of needles or needle-like objects into which defects are formed by omitting selected needles or needle-like objects. However, persons utilizing such intentional defects (that is, non-uniformly-sized holes or omitted needles) have not suggested any way the structures may be designed to effectively control losses.
One suggested device for reducing radiation losses is a planar PBG structure formed on a substrate that itself comprises a one-, two- or three-dimensional photonic bandgap structure. A significant disadvantage of such a structure is that it requires complex substrate preparation. Complex preparation steps necessarily offset another of the expected advantages that planar PBG materials usually have over three-dimensional alternatives: a relatively simplified manufacturing process.
The claimed structures are robust planar photonic bandgap structures. One structure is an improved planar photonic bandgap structure for controlling radiation loss, comprising:
(a) a patterned photonic bandgap region having at least one layer,
(b) an unpatterned substrate having at least one layer, and
(c) an unpatterned superstrate having at least one layer,
wherein at least some radiation loss is controlled by the thickness of at least one of the layers. That is, at least some radiation loss due to out of plane scattering is controlled by the thickness of at least one of the layers. In another embodiment, at least some radiation loss due to out of plane scattering is controlled by the refractive index of at least one of the layers.